An ultra high melt flow (UHMF) grade crystalline polymer has a melt flow (MF) of about 50 dg/min or greater. The MF of a UHMF crystalline polymer can be as high as 15,000 or greater. UHMF polymers in the range of about 1000-2000 are particularly useful for the production of non-woven fabrics by melt blown fiber processes. In order to employ UHMF polymers in commercial processing equipment, it is desirable to utilize the UHMF polymer as a pellet feed stock.
Pelletization of polymers using conventional pelletization systems is a well known method of providing a pellet feedstock. However, crystalline polypropylenes that have a MF of about 50 dg/min or greater are particularly difficult to pelletize. Due to the low melt strength of such UHMF crystalline polymers, attempts to pelletize UHMF polypropylenes with conventional pelletization systems, including underwater pelletization systems, result in an excess amount of non-uniform pellets, malformed pellets, pellet trash and high levels of "fines". Deformation of the polymer pellet is caused by water currents created by rotating knives of the underwater pelletization system. Malformed and non-uniform pellets are undesirable since they tend to bridge in pellet feed hoppers and convey poorly (e.g., plug conveying filters). Further, significant amounts of malformed pellets alter the bulk density of the pellet stock may result in feeding problems in the extrusion line and voids in the final product. In addition to malformed pellets, "trashouts" occur frequently during the production of UHMF crystalline polymers. Trashouts are extruder shutdowns resulting from polymer buildup on the rotating knives. Such trashouts not only necessitate the consumption of enormous labor and time but induce deterioration of the quality of polyolefin polymer pellets being produced.
It has long been desired to find a continuous process for pelletizing UHMF crystalline polymers to produce uniform, dust-free crystalline polymer pellets having narrow molecular weight distribution. In particular, it is desired to find a high speed continuous process for pelletizing crystalline polymers, such as isotactic polypropylenes, that have a melt flow greater than 50 dg/min.
Further, it is desired to find a process for pelletizing UHMF crystalline polymers that contain a uniform dispersement of the desired additives and are substantially cracked to produce uniformly compounded pellets having high bulk density.